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The cosmological tree theorem

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Salcedo, Santiago Agüí 
Melville, Scott 


A number of diagrammatic “cutting rules” have recently been developed for the wavefunction of the Universe which determines cosmological correlation functions. These leverage perturbative unitarity to relate particular “discontinuities” in Feynman-Witten diagrams (with cosmological boundary conditions) to simpler diagrams, in much the same way that the Cutkosky rules relate different scattering amplitudes. In this work, we make use of a further causality condition to derive new cutting rules for Feynman-Witten diagrams on any time-dependent spacetime background. These lead to the cosmological analogue of Feynman’s tree theorem for amplitudes, which can be used to systematically expand any loop diagram in terms of (momentum integrals of) tree-level diagrams. As an application of these new rules, we show that certain singularities in the wavefunction cannot appear in equal-time correlators due to a cancellation between “real” and “virtual” contributions that closely parallels the KLN theorem. Finally, when combined with the Bunch-Davies condition that certain unphysical singularities are absent, these cutting rules completely determine any tree-level exchange diagram in terms of simpler contact diagrams. Altogether, these results remove the need to ever perform nested time integrals when computing cosmological correlators.


Acknowledgements: We thank Harry Goodhew, Mang Hei Gordon Lee, Prahar Mitra, Enrico Pajer and David Stefanyszyn for useful discussions. S.M. is supported by a UKRI Stephen Hawking Fellowship (EP/T017481/1). S.A.S. is supported by a Harding Distinguished Postgraduate Scholarship. This work has been partially supported by STFC consolidated grant ST/T000694/1. Note added. In the final stages of preparing this manuscript, [115] appeared on the arXiv, which discusses cutting rules from a different (polytope) perspective. It would be interesting to investigate how the causality conditions and loop-level cutting rules presented here could be recovered from the optical polytope of [115].


Scattering Amplitudes, de Sitter space, Cosmological models, Effective Field Theories

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Journal of High Energy Physics

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Springer Berlin Heidelberg